Polylactide is a high-molecular compound in which a lactic acid or its derivative obtained from plant-based materials is polymerized by ester linkage. Due to its excellent physical and chemical properties as well as its biological compatibility and biodegradability, polylactide has been drawing attention as a high-molecular compound that is highly safe and causes little burden on living organisms or environments. In particular, polylactide, which is degradable and absorbable in living organisms, is expected to be used as a carrier in a drug delivery system (DDS) or biomedical materials, such as medical adhesives.
As compared to high-molecular petrochemical materials, polylactide has lower levels of thermal resistance and mechanical strength. Accordingly, studies for modifying the thermal resistance and mechanical strength of the polylactide have been conducted.
One of the conventional techniques is the stereocomplexation of the polylactide. A stereocomplex is a crystal structure which is formed by mixing L- and D-enantiomers and has the L- and D-enantiomers alternately and regularly arranged. A substance having such a crystal structure is also called the stereocomplex. In general, a stereocomplex has a higher melting point than the homocrystal of its constituents (L- or D-enantiomer) and has the potential to be a high-molecular material with high thermal resistance. Accordingly, the idea of mixing the enantiomers of the polylactide, i.e. poly-L-lactic acid and poly-D-lactic acid, to form a stereocomplex crystal and use it as a high-polymer material has been proposed (for example, see Patent Literature 1 or 2).
Another conventional method for modifying the nature of the polylactide is to introduce a substituent group at a terminal of the polylactide. For example, Non Patent Literatures 1 and 2 disclose poly-L-lactic-acid and poly-D-lactic-acid derivatives in which a caffeic acid derivative is chemically bonded to one terminal of the poly-L-lactic acid as well as one terminal of the poly-D-lactic acid. A stereocomplex composed of such poly-L-lactic-acid and poly-D-lactic-acid derivatives has higher pyrolysis temperatures, i.e. T10 (the temperature at which a 10-wt % decrease in weight is observed) and Tm (melting point), than its homocrystal. Thus, the thermal resistance has been improved.